The present invention relates to a system for controlling a lock-up torque converter of an automatic transmission mounted on a vehicle equipped with an anti-skid brake control system (a wheel slip brake control system).
Japanese Patent provisional publications Nos. 54-132062, 55-109854 and 56-127856 disclose lock-up torque converters having a lock-up clutch disposed between input and output members of a hydrodynamic drive to provide a direct mechanical drive. In a conventional automatic transmission having such a lock-up torque converter, the lock-up clutch is controlled by a control unit in accordance with a vehicle speed determined from an output signal of a wheel speed sensor or the like. A lock-up vehicle speed and a release vehicle speed equal to or lower than the lock-up vehicle speed are predetermined for each gear position of the transmission. When the vehicle speed exceeds the predetermined lock-up speed, the control unit makes the lock-up clutch engage by sending a lock-up control signal to a lock-up solenoid for switching hydraulic fluid passage in the torque converter. When the vehicle speed decreases to the predetermined release vehicle speed, the control unit deactivates the lock-up solenoid, so that the lock-up clutch is disengaged. In this case, there is some delay (normally, about 0.2 to 0.3 second) between the deactivation of the lock-up solenoid and the complete disengagement of the lock-up clutch because the lock-up clutch is operated by the hydraulic system. Consequently, there is a possibility that during hard braking the vehicle speed will decrease into an engine stall range before the lock-up clutch is disengaged.
Japanese Patent provisional publication No. 57-192668 proposes an lock-up torque converter control system arranged to prevent engine stall during hard braking. In this control system, the lock-up clutch is controlled in accordance with not only the vehicle speed but also the deceleration. If the deceleration exceeds a predetermined value, the control unit of this control system commands the lock-up solenoid to disengage the lock-up clutch even before the vehicle speed decreases to the predetermined release speed. However, this lock-up torque converter control system is not satisfactory when it is applied to a vehicle equipped with an anti-skid brake control system.